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Youths reportedly jumping off a highway bridge before one of three serious crashes on major Hawke's Bay roads in 15 hours has prompted a warning from police.

District road policing manager Matt Broderick said police had received more than 25 calls in recent weeks, reporting people climbing and jumping into the river off the Twyford Hawke's Bay Expressway bridge.

The bridge is narrow, has no pedestrian walkway, and the traffic speed limit is 100km/h.

On Thursday afternoon a truck driver was trapped in the cab of his vehicle after crashing on the bridge about 4.10pm.

A man in his 50s remained in Hawke's Bay Hospital in a stable condition on Friday afternoon, while three other male patients were assessed and later discharged.

While there was speculation that the crash was the result of distraction caused by youths leaping from the bridge, Boderick said no such behaviour was seen by police.

Police also questioned people below the bridge, but they denied any jumping activity.

Boderick said police strongly discouraged people from walking on to and jumping from the bridge, regardless of the cause of the crash.

Boderick said vehicles passing within a metre of each other on the bridge at 100km/h along with heavy vehicles travelling at speed could generate "significant air movement" which could potentially pull someone into the path of traffic.

"People walking on the bridge put not only themselves, but motorists at risk.